1. Field of the Invention
The present invention relates to disk drive heads for high track density perpendicular magnetic recording, and more particularly relates to a method of precompensating for non-linear transition shifts in data writing.
2. Description of the Prior Art
Present magnetic data recording systems such as disk drives record data on a magnetic recording medium such as a magnetic hard disk, as a series of magnetic field transitions, or changes in direction of magnetic polarity. Typically, the lack of a magnetic transition represents a binary “0”, while a magnetic transition represents a binary “1”. The magnetic write field is typically created by passing a current through a write head adjacent to the medium, creating a “write bubble” which defines a region where the magnetic field is sufficiently strong to be magnetically recorded on the medium. Magnetic transitions are created by reversing the direction of current flowing through the write head.
Until recently, data has been conventionally stored in a thin media layer adjacent to the surface of a hard drive disk in a longitudinal mode, i.e., with the magnetic field of bits of stored information oriented generally along the direction of a circular data track, either in the same or opposite direction as that with which the disk moves relative to the transducer.
More recently, perpendicular magnetic recording systems have been developed for use in computer hard disk drives. A typical perpendicular recording head includes a trailing write pole, a leading return or opposing pole magnetically coupled to the write pole, and an electrically conductive magnetizing coil surrounding the write pole. In this type of disk drive, the magnetic field of bits of stored information are oriented normally to the plane of the thin film of media, and thus perpendicular to the direction of a circular data track, hence the name.
Media used for perpendicular recording typically include a hard magnetic recording layer and a soft magnetic underlayer which provide a flux path from the trailing write pole to the leading opposing pole of the writer. Current is passed through the coil to create magnetic flux within the write pole. The magnetic flux passes from the write pole tip, through the hard magnetic recording track, into the soft underlayer, and across to the opposing pole, completing a loop of flux.
Perpendicular recording designs have the potential to support much higher linear densities than conventional longitudinal designs. Magnetization transitions on the bilayer recording disk are recorded by a trailing edge of the trailing pole and reproduce the shape of the trailing pole projection on the media plane, thus the size and shape of the pole tip is of crucial importance in determining the density of data that can be stored.
In both longitudinal and perpendicular write heads, there is a common problem concerning the standardized lengths of the regions in which the data bits are written. This problem is illustrated for the case where longitudinal recording is used in FIG. 2, and in the case where perpendicular recording is used in FIG. 3.
FIG. 2 shows the poles of a write head 6 over the surface of a magnetic recording medium, assumed to be a hard disk 24, as the disk moves in the direction of the arrow 7. The magnetic data bits on the left side are shown to have been established earlier as the write head 6 has written to them. For ease of reference, the data bits 3 have been labeled alphabetically as “a”, “b”, etc. The primary direction of the magnetic orientation of the data bits 3 is shown by the larger upper data bit direction arrows 4. It can be seen that when two consecutive data bit arrows point in the same direction (to the right in the figure) as do the first two arrows 4 in bit areas a and b, the data bit is a “0”. However when the consecutive arrows point in opposite directions, as they do in data bits b and c, where the second arrow points right and the third arrow points left, a transition has occurred, thus signifying a “1” data bit. Underneath the data bit direction arrows 4, are smaller D arrows 5 which show the presence and relative strength of a demagnetization field, to be called “D” for purposes of this specification. The D arrow 5 always is opposite in direction from the data bit direction arrow 4, and varies in strength, as indicated by the length of the D arrows. It will be noted that the length of the D arrow 5 in data bit a is larger than the D arrow 5 that in data bit b, although they have the same direction, since there is no transition present. It is commonly observed in longitudinal recording that the D field strength is greatest right after a transition (signifying a “1”) and decreases with subsequent non-transition data bits, i.e. subsequent zeroes, eventually reaching a steady state minimum value when a long string of zeroes is written.
A difficulty can result when the write head 6 writes a transition, signifying a “1”. In this case, the direction of applied magnetic field 8 will be opposite to that of the previous data bit arrow 4, but will be aligned with the previous D arrow 5 component. This additional D component thus adds to the field strength produced by the write head, and increases the size of the write bubble, making it expand from its normal extent to an increased extent. This is shown when the next data bit d is being written in FIG. 2. The data bit direction arrow 4 of data bit c points left, and the D arrow 5 points right. The applied magnetic field direction arrow 8 points to the right, so a transition is being written. The normal extent of the write bubble 9 is shown in the inner dashed line. However, due to the addition of the D field arrow 5, in the same direction as the applied magnetic field arrow 8, the write bubble is extended to make an expanded write field bubble 10. The extent of the expanded write field bubble 10 will determine where the transition boundary 12 will be positioned.
In this case, the transition boundary 12 thus is written farther to the left in the figure than that ideally transition boundary 13 positioned by the normal extent write field bubble 9. The actual transition boundary 12 is thus displaced from the ideal transition boundary 13 by some non-linear amount. This phenomenon is known as a “non-linear transition shift” (NLTS) in the magnetic transition pattern. These transition shifts can potentially cause errors in reading data from the disk, and can effectively limit the data recording rate of the disk drive to a level where the occurrence of transition shifts are sufficiently low to ensure accurate data recovery from the disk. The amount of the NLTS 16 is shown as the difference between the ideal transition boundary 13 and the actual transition boundary 12. As the actual transition boundary 12 is moved forward in time (left in the figure) compared to the ideal transition boundary 13, this type of NLTS is referred to as “positive NLTS” 17.
As a standard way of measuring the effect of NLTS, it has become common to look at a pair of transitions, where a “1” is followed by another “1”, with or without intervening zeroes. This transition pair is commonly referred to as a “dibit” 18. The period of the dibit 18, which in this case corresponds to data bit c, is thus decreased, as shown by the decreased period Td 15 of the dibit 18, compared in the figure with the normal period T 14.
As discussed above, in longitudinal recording, the magnitude of the D field decreases with distance from the last recorded transition. Thus the amount of NLTS is variable, with the largest effect being seen in successive transitions, and the effect lessening with each successive non-transition. Thus a pattern reading “1 1” would have a large NTLS, a pattern reading “1 0 1” would have less NTLS, a pattern reading “1 0 0 1” even less, and so on.
This pattern is reversed in perpendicular recording, shown in FIGS. 3–6. FIG. 3 shows the P3 pole tip 52 over the surface of the hard disk 24, as the disk moves in the direction of the arrow 7. The magnetic data bits on the left side are shown to have been established earlier as the write head has written to them. For ease of reference, the data bits 3 have again been labeled alphabetically as “a”, “b”, etc.
In perpendicular recording, as its name suggests, the direction of the magnetic bits is perpendicular to the disk surface, i.e. “up” and “down” in the figure. The primary direction of the magnetic orientation of the data bits 3 is shown by the larger data bit direction arrows 4 and to the right of the direction arrows 4, the demagnetization (D) field arrows 5 are shown in shorter dashed lines. Once again the magnetic data bits 3 on the left side are shown to have been established earlier as the P3 pole 52 has passed over them. Again it can be seen that when two consecutive data bit arrows point in the same direction (up in the figure) as do the first three arrows in a, b and c, the data bit is a “0”. However when consecutive arrows point in the opposite direction, as in c and d, a transition has occurred, thus signifying a “1” data bit, the same pattern as in the longitudinal recording.
As before, the D arrow 5 always is opposite in direction from the data bit direction arrow 4, and varies in strength, as indicated by the length of the D arrows 5. However, the difference with perpendicular recording is that the D field strength is smallest right after a transition, and grows larger with successive non-transitions, eventually reaching a steady state value when a long string of zeroes is written. Thus, it will be noted that the length of the D arrow in data bit a is smaller than that in data bit b, which is smaller than in data bit c, although they have the same direction, since there is no transition present.
Once again, when the P3 pole 52 writes a transition, signifying a “1”, the direction of applied magnetic field 8 will be opposite to that of the previous data bit arrow 4, but will be aligned with the D arrow 5 component of data bit e. This additional D component thus adds to the field strength produced by the P3 pole 52, and increases the size of the write bubble, making it expand from its normal extent 9 to an increased extent 10. The transition thus is written farther to the left in the figure than that ideally positioned by the normal extent field 9.
As discussed above, a dibit 18 includes a pair of transitions, where a “1” is followed by another “1”, with or without intervening zeroes. The period of the dibit 18, which in this case corresponds to data bits d and e, is thus decreased, as shown by the decreased period Td 15 of the dibit 18, compared in the figure with the normal period T 14.
As referred to before, the D field component increases with successive non-transitions, and thus the amount of NLTS is variable, however with the smallest effect being seen in successive transitions, and the effect increasing with each successive non-transition. Thus a pattern reading “1 1” would have almost no NTLS, a pattern reading “1 0 1” would have a small NTLS 16, as shown in FIG. 4, a pattern reading “1 0 0 1” having more NTLS 16, as shown in FIG. 5, and a pattern reading “1 0 0 0 1” having even more NTLS 16, as shown in FIG. 6.
The NLTS phenomenon has been observed and accounted for in prior art magnetic recording systems by a process known as precompensation. Precompensation attempts to adjust the timing of the current transition bit to ensure that the transition is located properly on the medium, compensating for the effect of the demagnification field of the previous transition bits on the write bubble field used to record the current transition bit. Write precompensation is commonly used in longitudinal magnetic recording systems. The demagnetization field in longitudinal recording is maximum in the vicinity of magnetic transition. This field causes non-linear transition shift for transitions, recorded at high linear density, so as the recorded closely spaced transitions are shifted “early”. The standard write precompensation method is utilized in longitudinal magnetic recording channels, applying “late” delays of magnetic transitions, preceeded by another transition.
This precompensation strategy can not be applied to perpendicular recording. As discussed above, the demagnetization field in perpendicular media is small at the transition vicinity and increases with distance. This is opposite to the longitudinal media and causes maximum transition shifts for relatively isolated transitions. High density transitions (successive “1”s) are not distorted by NLTS (K. Senanan, R. Victora “Theoretical Study of Non-Linear Transition Shift in Double Layer Perpendicular Media”—IEEE Trans. Magnetics, vol. 38, 42002 pp., 1 Combination of these effects causes “negative” NLTS, which was experimentally measured in perpendicular recording media. Therefore, the precompensation method for perpendicular recording is to be modified, compared with longitudinal recording case.
The straightforward solution for precompensation of perpendicular recording channels is to use “negative” precompensation, i.e. shift all high density transitions “early” in time. However, it is believed that this method has not been implemented in practical systems. Negative precompensation of high density transitions may present technical difficulties at high data rates, requiring effective increase of channel frequency. Also, the negative precompensation does not allow more complicated precompensation schemes, having more than one level of timing shifts and providing better control of total non-linear distortion
Thus there is a need for a precompensation system for perpendicular write heads that can compensate for the NLTS effect without the disadvantages of negative precompensation, and which can allow multiple levels of delay to account for variable NLTS.